1. Field of the Invention
This invention relates to polyester containers having a reduced coefficient of friction and methods for making such containers.
2. Description of the Prior Art
Problems exist in conveying various types of polyester containers due to the excessive amount of static friction encountered when container surfaces contact. This excessive friction can lead to “process line” or “filling line” interruptions that are economically undesirable. The problem occurs after the polyester polymer has been molded into preforms or stretch-blown into various types of containers. The containers are sometimes conveyed directly into a palletizing station and then shipped to a filling plant or they are conveyed to a labeling and filling line contained within the same plant. This problem is more pronounced in the carbonated softdrink (“CSD”) industry due to the high speed of stretch-blow molding conveying and filling lines. The problem is also encountered in other parts of the polyester container industry where the containers are being conveyed under pressures applied from congested areas of the conveying process.
During the process of blowing and filling stretch-blow molded polyester containers it is necessary and common to convey the containers along conveyor belts or rails. For example, the containers are typically moved from stretch-blow molding machines to a palletizer and loaded in some formation such as 15×15 array onto a cardboard layer. Then, the layers are stacked several layers high before the entire stack is shrink-wrapped for shipment to a filling line. Alternatively, the containers are depalletized by taking them off the pallet and moving them onto a conveyor line and through the labeling and filling process. During these processes, the containers have a tendency to stick together and cause line jams as they proceed to the filler or labeler or stick together and cause gaps in the formation required for the palletization process. Also, the pressure between the individual containers is at its greatest and any gaps that form are hard to eliminate due to these pressures and the friction between the containers.
Certain container types (e.g., two liter poly(ethyleneterephthalate) (“PET”) CSD bottles) are essentially straight-walled and have a very smooth surface that gives the container an appealing appearance. However, the very smooth, flat surface of the container maximizes the surface which comes in contact between two adjacent containers. With the inherently high COF polyester containers such as PET (PET has a static COF greater than 1.0), the containers become entangled and “tip over” or just stop moving in the conveying line after blowing, during filling, enroute to the palletizer, or enroute from the depalletizer to the labeling and filling station. Such tip over and stoppage obviously causes undesirable disruptions in the conveying or filling process.
A high COF prevents adjacent containers on a multiple-row conveying line from moving (turning or slipping) during conveying. When the conveying line changes direction, sometimes as much as 90 degrees, the containers may become entangled and either stay upright and stop the feed or tip over and stop the line. In either event, someone has to monitor these problem areas at all times to keep the line moving. Therefore, a container having a low static COF that could slide and rotate against other containers during conveying would minimize or eliminate process downtime and the need for someone to constantly monitor the process.
There exists prior art in the area of thermal crystallization of the preform and bottle prior to and during the stretch-blow process. However, such art does not disclose any reduction in the bottle sidewall COF nor any improvement in “bottle stickiness.” JP 3207748 and JP 216081 disclose adding a small amount of polyamide nucleator to improve crystallization throughout the entire thickness of the bottle during the heat-set process to improve thermal stability. U.S. Pat. No. 5,090,180 discloses crystallizing the entire thickness of the base by thermal means during the stretch-blow process to improve thermal and mechanical stability of the bottle. JP 62030019 discloses reducing internal residual strain in a two stage stretch-blow process by thermally crystallizing the entire bottle before the second stretch-blow step, yielding a bottle with a low degree of haze. JP 58119829 discloses passing the preform through a flame treatment to melt the surface, causing some thermal crystallization, and reducing surface defects without imparting haze.
There is prior art in the area of solvent crystallization of PET to improve the thermal stability of PET bottles. However, this art does not disclose the use of solvent crystallization of the preform or bottle surface to decrease the container sidewall COF. JP 56150516 and JP 53110669 disclose that the neck and mouth of the bottle, after the stretch-blow process, can be solvent crystallized to improve solvent-crack resistance in the bottle without increasing the haze level in those regions.
None of the above cited prior art disclose selectively treating only a portion of the preform or container wall to reduce the COF and improve the handling properties of the container. There is, therefore, a need for new and improved containers having a reduced COF, particularly low haze (high clarity) containers that have a reduced COF, and for processes for producing such containers.